They are kind of different, Im pretty sure the LAV-25 is lighter than the Grizzly, because is a recon vehicle while the grizzly an APC. They probably took armor off of it so it would be faster, but still have a autocannon.

But it also may be apart of the nerf they got after someone complained that they were OP

LAV also is designed to go in water. It is a very light skinned vehicle. The angles on the surface do more to stop incoming rounds than the actual thickness of the skin itself. Well placed .50 caliber round can penetrate the LAV's hull.